In the field of producing semi-conductors, a large amount of liquid chemicals and water have been conventionally used in wet processes. A fluorine-containing resin having excellent chemical resistance, heat resistance and melt moldability is used for pipes transporting such liquid chemicals. Among fluorine-containing resins, a copolymer (PFA) of tetrafluoroethylene (TFE) and perfluoro(alkyl vinyl ether) (PAVE) and a copolymer of TFE and hexafluoropropylene (HFP) are excellent in chemical resistance, heat resistance, melt moldability and stress cracking resistance, and thus preferably used for piping arrangements such as tubes and joints for transporting liquid chemicals.
However, these PFA or FEP piping arrangements have a problem of chemical liquid permeation in a small amount and needs some improvement. In current semiconductor production plants, there are some countermeasures such as covering the outside of a tube with a pipe made of poly(vinyl chloride) (PVC) to make a double piping structure, or in the wet station area, exchanging tubes regularly, wrapping them with a PVC film or further wiping the outside of a tube with a cloth.
These measures increase equipment and maintenance costs as might be expected, resulting in the increase of the cost for producing semiconductors.
As mentioned above, it is now practically difficult to solve liquid chemical permeation problem of PFA or FEP in view of structure and maintenance, and therefore improvements are investigated with respect to its materials.
For example, there is a measure to increase crystallinity of PFA or FEP. It is effective to increase crystallinity thereof since permeation of liquid chemical is generally observed at amorphous parts. It is possible to increase crystallinity by decreasing the amount of PAVE in case of PFA and HFP in case of FEP. In this case, however, there arise defects that processability and crack resistance are decreased.
In addition, Japanese Unexamined Patent Publication No. 116706/1999 discloses a process for irradiating a molded article of a fluorine-resin such as PFA or FEP with ionizing radiation under inert gas atmosphere at temperature of at least the melting point to cross-link the polymers, thereby improving gas barrier property. However, special apparatus is required for such treatment, and this is not preferable from an economical point of view.
Alternatively, there is a measure to use a fluorine-containing resin more impermeable to liquid chemicals, and among the fluorine-containing resins, it is effective to choose poly(chlorotrifluoroethylene) which is the most impermeable to liquid chemicals. However, this resin has a problem that it is poor in stress cracking resistance, moldability and heat resistance.
Furthermore, modification of FEP is also proposed. For example, Japanese Examined Patent Publication No. 45084/1980 discloses modification of FEP by using particular PAVE, thereby improving tensile strength and stress cracking resistance at high temperature. PAVE used for the modification is lower PAVE with alkyl group having at most 4 carbon atoms, which shows no prevention effect on liquid chemical permeation.
Japanese Unexamined Patent Publication No. 210941/1999 also discloses production of a tube using a terpolymer comprising TFE, HFP and PAVE, in which sphaerites of FEP is made smaller. PAVE used in this case is also lower PAVE with alkyl group having at most 4 carbon atoms, which shows no prevention effect on liquid chemical permeation.
An object of the present invention is to provide a melt-moldable fluorine-containing resin material which can provide, in an economically efficient manner, a molded article having excellent impermeability to liquid chemicals while retaining excellent processability, heat resistance and the like inherent in FEP and a molded article using the same.
As a result of intensive studies, it has been found that chemical impermeability can be improved without losing inherent properties of a resin by modifying FEP using a particular perfluoro(alkyl vinyl ether) and the present invention has been completed.